The present invention relates to conductive, thermoset molding compositions and articles made therefrom. More specifically, the present invention relates to molding compositions containing thermoplastic additives comprising conductive carbon black which can impart electrical conductivity to articles molded from the molding compositions.
Thermoset molding compositions are commonly used to make a variety of articles such as, for example, automotive body panels, truck cab components, appliances, home furnishings, e.g., bathtubs, doors, and the like. Such molding compositions typically comprise an unsaturated thermosetting resin, an olefinically unsaturated monomer which is copolymerizable with the thermosetting resin, a thermoplastic additive, fibrous materials, and various other ingredients, including for example, fillers, mold release agents, and the like.
In some cases, the molding compositions comprise a pigment which imparts a desired color to the molded article. In other cases, the molded articles are painted in order to provide the desired color. Electrostatic spray systems are being used more frequently in the industrial application of primers and paints because of improvements in the transfer efficiency, that is, the amount of primer or paint deposited on the article versus the amount of primer or paint sprayed. In systems which do not utilize electrostatics spray processes, xe2x80x9cbounce backxe2x80x9d and xe2x80x9coversprayxe2x80x9d can reduce the transfer efficiency. Bounce back occurs as atomized droplets of paint approach the surface of the article and an air current rebounding from the same surface deflects the droplets into a new trajectory away from the article. Overspray occurs when a portion of the paint is sprayed beyond the article""s edge into the surrounding space.
In electrostatic spray processes, atomized paint droplets are passed through an electric field charging the droplets. The droplets are drawn to the article to be painted which is at ground potential. If the article cannot conduct electric current, then the article becomes increasingly charged. This charge buildup on the articles"" surface eventually repels new incoming charged droplets. This repulsion can lower the transfer efficiency quite dramatically. Therefore, it is highly desirable in electrostatic paint processes that the surface of the article be made conductive.
Both sheet molding compound (xe2x80x9cSMCxe2x80x9d) and bulk molding compound (xe2x80x9cBMCxe2x80x9d) are good insulators and have poor electrical conductivity. SMC and BMC molded articles can be made conductive by incorporating a conductive material, e.g., carbon black, into the molding composition or by applying a conductive coat (known in the art as a xe2x80x9cdust coatxe2x80x9d) on the molded article. The incorporation of carbon black in its neat form into molding compositions is generally undesired in the art because handling carbon black presents significant housekeeping problems and health concerns. Applying a dust coat is also generally undesirable because it adds an additional step to the painting process which often increases costs.
Accordingly, improved molding compositions are desired to provide electrically conductive molded articles which are suitable for electrostatic painting. Desirably, the improved molding compositions would allow molders of articles to avoid the necessity of handling neat carbon black and avoid the use of a dust coat in the electrostatic painting process.
By the present invention, improved molding compositions suitable for manufacturing electrically conductive articles are provided. As a result, articles molded from the molding compositions of the present invention can be painted by electrostatic painting processes which can lead to enhancements in the transfer efficiency of the paint to the article, i.e, a reduction in bounce back and overspray and a corresponding reduction in the amount of volatile organic compounds released to the environment.
In accordance with the present invention, electrically conductive, carbon black is incorporated into compositions comprising a thermoplastic polymer (hereinafter referred to as xe2x80x9cconductive additivexe2x80x9d). The conductive additives are preferably prepared in the form of pellets or extrudate prior to their addition to the molding compositions. Quite advantageously, the incorporation of the conductive carbon black into the conductive additive avoids the necessity of handling carbon black in its neat form. Also, the amount of carbon black introduced into the molding compositions can be more accurately controlled.
Quite surprisingly, it has been found in accordance with a preferred aspect of the present invention that by adding a lubricant, e.g., zinc stearate, during the manufacture of the conductive additives, the electrical conductivity of the conductive additive and the resulting molding composition in which it is incorporated, and articles molded therefrom, can be enhanced.